Alternating-signal generator



' Jan. 24', 1961 J. MARLEY ALTERNATING-SIGNAL GENERATOR 2 Sheets-Sheet 2 Filed Sept. 3, 1957 Time FIG.2

United States Patent 2,969,470 ALTERNATING-SIGNAL GENERATOR John Marley, Fort Wayne, Ind., assignor to Hazeltine Research, Inc., a corporation of Illinois Filed Sept. 3, 1957, Ser. No. 681,674

7 Claims. (Cl. 307-885) GENERAL The present invention is directed to alternating-signal generators such as sine-wave generators which are particularly adapted for use in transistorized television receivers.

Transistors offer a number of advantages over electron tubes and their use has become rather prevalent in radio apparatus such as portable radio receivers. Transistors are characterized by their especially small size, lightness of weight, ruggedness, long operating life, extremely simple power requirements, and zero warm-up time. As a result, designers of television receiver equipment are giving transistors widespread use wherever possible in television receivers. I

The present state-of the transistor art-indicates that television receivers of the near future will be of a hybrid design such that some sections of the receiver will use electron tubes while others will employ transistors. Op-

employed in line-scanning systems for deriving-boththe' scanning wave and the voltage just mentioned.

Applicants copending application Serial No. 662,517, filed May 29, 1957, now abandoned and entitled Energy- Supply Apparatus, and application Serial No. 662,516, now abandoned filed concurrently therewith, and entitled System for Generating A Periodic Scanning Current, disclose and claim a power-supply system and a line-scanning system employing junction transistors for use in transistorized television receivers. For their operation these systems require an alternating-signal generator which is responsive to the line-synchronizing pulses for developing a sine wave having the line-scanning frequency. These line-scanning pulses have a duration whichis about 8 percent of the scanning period and hence have an energy content so low that it is quite difiicult to develop a satisfactory sine wave therefrom withouttaking special measures. The alternating-signal generator of the present invention is particularly adapted to develop from the line-synchronizing pulses a sine wave for application to the transistorized systems of the type disclosed in applicants above-identified copending applications.

It is an object of the invention, therefore, to provide a new and improved alternating-signal generator employing a junction transistor.

It is another object of the invention to provide a new and improved alternating-signal generator suitable for use in conjunction with the line-scanning or voltage 2,969,470 Patented Jan. 24, 1961 supply system of a television receiver employing transistors in the system just mentioned.

It is a further object of the invention to provide a new .and improved sine-wave generator which makes use of the holeor carrier-storage phenomenon occurring in junction transistors.

In accordance with a particular form of the present invention, an alternating-signal generator comprises a junction transistor having input and output electrodes and exhibiting a carrier-storage characteristic at a predetermined" frequency and energizing means coupled to those output electrodes. The generator further includes means coupled to the aforesaid input electrodes for rendering the transistor normally nonconductive, and additional means coupled to the input electrodes for applying thereto periodic pulses having the aforesaid predetermined frequency. The pulse duration intervals are much less than the pulse separation intervals, and the polarity of each pulse causes the transistor to conduct and store carriers during the pulse duration. The carrierstorage characteristic of the transistor is such that the stored carriers flow out therefrom to maintain it conductive during a much longer portion of each subsequent pulse separation interval. The alternating-signal generator additionally includes a load circuit coupled to the output electrodes and including a circuit resonant at a frequency related to the aforesaid predetermined frequency and responsive to the stored carriers for developing an alternating signal having the above-mentioned resonant frequency.

For a better understanding of the present invention, together with other and further objects thereof,'reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring to the drawings:

Fig. 1 is a circuit diagram, partly schematic, of a complete television receiver including an alternating-signal generator in accordance with a particular form of the present invention, and

Fig. 2 is a graph utilized-in explaining the operation of the Fig. l alternating-signal generator.

, General description of receiver of Fig. 1

Referring now more particularly to Fig. l ofthe drawings, the television receiver there represented comprises a receiver of the superheterodyne type including an antennasystern 10 coupled to a radio-frequency amplifier 11 of one or more stages. There are coupled to the latter in cascade, in the order named, an oscillator-modulator 12, an intermediate-frequency amplifier 13 of one or more stages, a detector and automatic-gain-control or AGC unit 14, a video-frequency amplifier 15 of one or more stages, and cathode-ray tube image-reproducing device 16 of conventional construction provided with the usual line-frequency and field-frequency scanning wind- 1 ings 18 and 19, respectively, for deflecting the cathoderay beam in two directions normal to each other. The AGC supply unit 14 is connected to the input circuit of at least the first stage of the intermediate-frequency amplifier 13 when the latter includes transistors as will be mentioned hereinafter. Connected to the output terminals of the video-frequency amplifier 15 is a conventional intercarrier sound-signal reproducing system 20 which includes the usual 4.5 megacycle signal selector, frequency detector, audio-frequency amplifier, and loudspeaker.

The output circuit of the video-frequency amplifier 15 is coupled through a synchronizing-signal separator 21 to the input circuit of a field-frequency generator 22 and to input terminals 70, 70 of an alternating-signal or sinewave generator 23 in accordance with the present in vention. The output circuit of the field-frequency gen,-

13 erator '22 is coupled to 'thefield scanning winding *19 while output terminals 87, 87 of the alternating-signal generator 23 are coupledto a pair of input terminals 2 5, 25 are combined energ' "supplyand lin scannin system 24. The portion of the latter"virhich'is" presented inth'e drawings is constructediiraccordance with the teachings ideiitifiedl'dfiehdingapplications andreference"is"fna'de thereto for complete detailsfoffls'tructu're and operation Briefly, however, system 24 includes a junction'lransi's'for 32, such as a type 2N174 alloy-juncs'istoi', having "emitter-base input elec- I I ls'25, 2S"ar'id eonectayemitter 'outpiit .el eetrod'es connected in a'm'a'nner to he described's uhsfeqiieintly. System 24 comprises aloadcirvtillit 33 includinghniailtotransforiner 34 havingjla plural- 'ity ofwin'din'g pe sons 35, :36, swans 3s. The linejs'cannihglvv indirig' 18 is connected to the Winding portion 36 through a paino ffjo'utputterminals"26, 26 and connectionsioliffill. Condenser 39,which may be in whole or in part the inherent capacitance associated with the transformer windingsandhence is represented in broken lineconsitruct'ion, is connected in parallel with the po'rtjion is resonant with the line-scanning winding :18 at a frequency" several times that of the line-scanning frequency. load also comprises a directcuri'e'nt circuit vvhich includes ,a unidirectional supply means nepre'sented as a source E of about 12 volts, the W hh hs 1215 3 6 3nd' .3I a uhidirectiohally hondmtirehevi' s which is fil m qm iQr u n, 9 I40 hhhsstesl hht h le etehh h Wind n andr- !;-.5 t ltet hhde Th eysfi d hstp t n 35 pr e ably includes a few turns for developing a small potential h Q e t t vQs-MLTIAQiuh t qnz f p 5 A I 35 and 36 is -connected to the'col- 'lector electrode of trar sis tor 32, I v p v The load circuit 33 also includesa potential-supply syste w ich ,ihthrh inc ud s a w g 43 n u y co p ed; t h tprev ous v ide fied wi d ng. P 36 andiwhi hza sc ihclndsa uh r c i h l y. conductive evice or diode 44 responsive to thepotential developet l in r etw hdihgnl dmiugtmce i t r ls of the aw-1 current developed twindi g 13 for der vi g a fir un directional potential having a value v djfierent from that of the supply -nsf fE one t ermifl'ahofthe winding "43is "g'i'dhnded While '-the other "term inal is connected thi'oi1ghthe J'dihde j-44 to an -"output termiii'al' 29 land to gr nndtthrongh ifilt r'condensef 46. The turns-tanner the W1 diii'g' 43"a'nd" the vv'iriding portion 36 is such that "there is' d'evlbped'afthe terminal 29a'v'oltage +B of -Imaghithdhuitiiblefbrapplication to tlie'terininal +3 of the radio-frequency amplifier 11 and the oscillator- "moaurator 1'2. 'l hepot'ential-s'iipply system '42 ma also includeanotlier re'c'tifierar'raiigement similar to the one just describedfor developing a" voltage '|B for application by terminal 3t to"theterrnifial +B 'of the videov. plied. s .St rl y h sys m, Zlhl eh t 1-1ih scanning waves and "the operating potentials for the first J ediQd A onducti e dhj e temtt Th d o e. .40. is.

"and second "anodes of the-image-reproducing-device 16.

The circuits for developing these anode operating potentials of the device 16 will he described hereinafter.

The load circuit of system 24 further includes a high voltage recetifier system including a diode 54 having its anode connected to the high voltage terminal of winding portion 38 and its cathodeconnected to ground through a storage condenser 561and' to an 'output terminal 28 h chl hp l s a hi h. nis hqti hh P. h h1 to t onda'node of the =i mage resprqducingjrl'eyice'16 An'auxiliary transformer ivinding 55 supplies current to the filament of thepdiode *54. Q'Anoperating potential for the accelerating electrode of device lfi is developed by a pcak-rectifyin'syste'm 'vvh'ich includs a" crystal diode 51 having one terminal connected to the junction of the winding portions 37 and 38 and its other terminal connected to an output terminal 27 and to ground through a filter network 52, 53. The rectifier 51 is poled to conduct the positive-going phlses developed during retrace intervalsof the'scanning wave suppliecl'to the-line-scanning winding'of'thedevice 16.

The units 10-16, inclusive, and 20-22, inclusive, may be"of:conventional. construction and *operation so that :adetailed description and explanation'of'the operation thereof are deemed unnecessary herein.

fieife'r al' opratidnof Fig. 1 receiver Considering briefly the general operation of the abovedescribed receiver as a whole, signals intercept ed by the antenna System10 are selected and amplified in the radiofrequency amplifier 11 and are supplied to the oscillator- ,modul ator;1f2 wherein they are converted to intermediateeqii hhyf ish lst -Ihe la ertxihfihm s h iycl lt mlll fifidl y th ih hzcrlia rh hehhyhmihi s fihh ar si liv et -.de ehthh and.h tqhi tiesaih-hhhhh ,h hpn m t. 4-"1Tb?.QQQPJ li ELQ E PQN QP92 V-t i h l-. sr erivedb thedht thrpof u i ilhhdh eshhp d to the v e req ency. h l fi hli (W i l he are then amplifiedyand supplied to the hrillianevcorrtrol e t d of the eg p dhc sd v hhl- .IAQWP ,voltagez e e rby the h hm ehiheh hllh RPlLQ un 4- .app i stasn aht hh ihhmhl hht hhh t n the e .h:h .h 9 r it t fifi tl n e ed a ration thereof. "Generator 23 develops V p 1 ltage atline-scanhing'frequency forappli- M cation tofthe 'eorhhi ried "l" annin g and energy-supply syste rn 'z i to'oiit'rfolthe 'operatron'thereof. Operating'po- "'tjii'tialsforfiinitsi 11, ,1: an 1515114 designated as "+13, m [1 system 24 at te'rminals 2 9 and a r i ht hdfihns d e a i nsa' l st m24, e ens t t r a t x 18 h .vh hi sflh 'vd taeelh h s f ppl qaproducing 'devicejlfi An electron the cathode-ray "i agere qemin Beam is ontrolled r the s geener the'tuhe and therebyrecon'struct hht h sh hh hre The sound signalisapplied by t he video fi eduiicy' am- $9 hhhh hzhe ether a leo plifier 15 to the sound-signal reproducing system 20 wherein the audio-frequency modulation components are derived in a conventional manner by the frequency detector of that unit and are then applied to the audio-frequency amplifier thereof for amplification and application to the loudspeaker for conversion to sound.

Description of alternating-signal generator 23 of Fig. 1

Referring now to the alternating-signal or sine-wave generator 23 of Fig. 1, that generator comprises a junction transistor such as a PNP germanium alloy-junction power transistor 73 having the usual input and output electrodes and exhibiting a carrieror hole-storage characteristic at a predetermined frequency such as at the line-scanning frequency of the television receiver. More particularly, the transistor 73 has emitter-base input electrodes and collector-emitter output electrodes. The transistor is represented diagrammatically as having an emitter region 74, a base region 75, and a collector region 76, the emitter region being grounded while the collector and emitter regions are interconnected through the primary winding 81 of an impedance-matching transformer 79 and through an energizing means represented as a source or battery -E, the positive terminal of which is grounded.

The sine-wave generator 23 also includes means coupled to the emitter-base input electrodes of the transistor 73 for applying thereto periodic pulses having the aforesaid predetermined or line-scanning frequency, durations much less than those of the pulse separations of the periodic pulses, and such polarity as alternately to render the transistor conductive and nonconductive alternately to store carriers in the base region 75 of the transistor for intervals much greater than at the aforesaid durations and to remove carriers from the base region. This periodic pulse applying form comprises the input terminals 70, 70, one of which is grounded while the other terminal is connected to the base region 75 through a base-current controlling resistor 78. The ungrounded one of the terminals just mentioned is connected to ground through a resistor 77 which with resistor 78 and the input impedance of transistor 73 may effectively match the output impedance of the synchronizing-signal separator 21.

The generator 23 further comprises a load circuit 80 which is coupled to the output or collector-emitter electrodes of transistor 73 and includes a circuit 86 resonant at a frequency related to'the above-mentioned predetermined frequency and responsive to carriers stored in the base region 75 for developing an alternating signal having the above-mentioned resonant frequency. The load circuit 80 includes the impedance-matching transformer 79 with its primary winding 81 and step-down secondary winding 82 and the resonant circuit 86. The latter comprises a condenser 85 together with an inductor 83, a small resistor 84, and the small base-emitter resistance of the transistor 32. For the television application under consideration, the tuned circuit 86 is resonant at the line-scanning frequency although for other applications it will be understood that its resonant frequency may be n times that of the frequency of the pulses applied to the input terminals of the circuit where n is an integer. Thus, for some applications the tuned circuit 86 may be resonant at a harmonic of the signal applied to the input terminals of the generator 23. The condenser 85 has a relatively large capacitance and is connected in parallel with the secondary winding 82 of the transformer 79. The output terminals 87, 87 of generator 23 are connected directly to the input terminals 25, 25 of unit 24, one terminal of each of the pairs being grounded. The tuned circuit 86 effectively comprises a series-resonant circuit which includes resistive elements affording a very low resistive impedance and this tuned circuit has a relatively high Q in the range of 8-10.

6 Operation of generators 23 of Fig. Iv

In considering the operation of the generator 23 of Fig. 1, reference is made to the curves of Fig. 2 which will be helpful in understanding the'operation of that generator. PNP type, negative polarity pulses represented by curve A of Fig. 2 and occurring at the line-scanning frequency are applied to the input terminals 70, 70 of generator 23. A junction power transistor such as the type 2Nl58 underconsideration exhibits a carrier-storage characteristic at the line-scanning frequency. During the pulse intervals 10-11, t t and t t-;, the transistor 73 is driven sufficientlyhard that the collector current rises to saturaion and carrier storage takes place in the base region 75. When the pulses of curve A end at times 1 t and t the carriers developed in the region of the base-collector junction are not completely swept out of the base zone and hence continue to drift across that junction for a considerable interval of time until they are depleted. The collector current of the transistor has the wave form representedby curve B of Fig. 2 and it will be seen from the shape of the current wave during the intervals t t t t and t -t that the collector current decays linearly until it ceases altogether at times t and t,,. The durations of the pulse intervals t --t t t and ts-tq are approximately 5 microseconds whereas the durations z t t t and t t of the pulses of collector current represented by curve B are between 20 and 30 microseconds.

The current pulses of curve B effectively drive the tuned circuit 86 which is resonant at line-scanning frequency and develop therein the sine wave represented by curve C of Fig. 2. The peak-to-peak amplitude of this sine wave developed across an impedance of a few ohms is several times that of the amplitude of the negative polarity pulses of curve A applied to the input terminals 76, 70. Input pulses such as those of curve A, if applied directly to the tuned circuit 86, would have insuflicient energy content to excite the tuned circuit. However, the transistor 73, by virtue of its high power-handling capabilities and high amplification coupled with the carrierstorage characteristic which it exhibits at the line-scanning frequency, impresses the substantial signal of curve B with its large duty cycle on the tuned circuit 86 so that it, in turn, develops a large sine-wave signal for application to the input terminals 25, 25 of the combined linescanning and energy-supply system 24. Consequently, the latter is caused to function satisfactorily to develop the several unidirectional potentials of various magnitudes for application to various units of the television receiver in addition to deriving a saw-tooth current wave for application to the line-scanning winding 18 ofthe cathode-ray image-reproducing device 16.

While applicant does not wish to be limited to any particular circuit constants, the following have proved to be useful in the sine-wave generator 23 of Fig. 1:

' Both on 1-inch diameter re-entrant core leg of enclosmg ferrlte transformer core -E -10 to --12 volts Transistor 7 3 CBS type 2N158 PNP germanium alloy junction power transistor Since the described transistor 73 is of theh While thfe has aeenueseiibedwrara atpresent connideied toilzk the preferred embodiment of this---invention, itWilFbobvibUs to those skilled i'n the art-that various fianges ndmodifications-mama made therein without v de arting from the invention, and it is, therefore, 1 aimed fto cover all such changes {and 'niod-ific-a'tions as fall within the" true spirit and" 's'eope' of the invention.

' -V\l iat--is claimed is:

1. altrnatin '-s'ignal-" 'geiieiat'r" dniprisiiig: a junc- V iha' iig inputf d olitput electrodes'and exhibiting a arrier' orage h aeteris'tic'at-a predeterr'n'ined nequeae'y nerg-izin'g means coupledto fsaid output electrodes "rr'ie'ans eoupld to -said input electrodes dor inderin'g aid istor 5 n inally nonconduetive; further-means" coupled-to"- said iiiput electrodes for 'applyi g thereto periodic "pulses hzivifig -said predetermined r-raquene the pune duration intervals-being much less than the pulsesepai'ation intervals an'd 'the polarityof eaeh pulse causing said transistor to conduct andstore carriers durin'gs the "pulse: diiiation; 'the earrier storage e'haract'eri's'ticof said? a-nsi's'tonheingsuh-thatthe stored carriersfioW-out there our to' ina-inta'in it conductive dur- "ing a much-longer-po "n-of' eaeh subsequent p'ulseseparationinterval; and a load'circuit"coupled to said output "electrodes and including-"'acirc'uit resonant at a frequency 1 related to said predetermined frequency and responsive to i said stored carriers for developingan' -alternating sigm1 =having s'a'id- 'resonant frequency.

2. An alternating-signal generator comprising:--a'iunc ti'on transistor-havinginane-and output electrodes and" exhibiting a barrier-storage characteristic ata predeterinined frequ'eric'y; energi' ng iiie'ans coupled -to saidottput electro'desfniea" pied to s'aidinpuf electrodes for renderi'ng "':ia.id transistor norrnallynonconduetiveffurther means coupled to said i'nput electrodes forapplying thereto periodidpulses having said predeterminedirequency, "the "pulse-duration intervals being muchless than the pulse "separation intervals and l the polarity of "each pulse causingsaid transistor to conduct-land store carriers during the 1 pulse duration? the carrier-storagecharacteristic of said transistor heing '{such "that the stored carriers flow a out therefrom to n'iaint'ain i-t conductive during 1 a 'mu'chlonger-portin o each subsequenrpuise separation iiit'e1"val;" and a' lea uit coupled to said output elecci'rcuit esonanea a frequeney n wheie fls' an integer, rs for developingan iiliibiti'n'g -a A cameie'mirge characteristic {at i a predetermined"-frequenc yf energizing-means coupled to said output electrodes? means-' coupled" to szfid' input electrodes itor rendering -'-s aid[ti'-ans'is'tor normally 'nonconductive; 5 furthermeans e'eupled o said input'electrodesfor applying thereto periodic pulses having' saidpredetermined frequency, the pulse duration intervals beingmuch less than the" pulse separtionjntervals and the polarity of each F pulse causing said transistor to conduct and store carriers during the pulseduration; the carrier-storage chame- "teristic of said transistor being such that the stored carriers flow out therefrom to maintain it conductive during a much longer portion of each subsequent"pulseseparaftion i'rit'i'valfand"afloadcifcuitcoupledto said output electrodes and "meluuingwaureuit resonant at' 'a'fr'equency equal to said predetermined frequeneye'nd'rs'ponsive to stored carriers" forue'veloping an alternating signal h iiig s'aid' resonant frequency.

""4 s'ifie wave' geaerator'ror"a'relevisionreeeiver compn ingfa junction transistor having input and output "-61ec trodes and" eirl:iil5itii1g-a ca'i-ti storage characteristic at line-scanning frequencyfenergizing? in'eans coupled to said output electrodes; means coupled to said input electrodes for rendering s'aid -transistor normally noncondncapplyiiig "theito"priddicpulses having said line 'se'ain- 'ning frequency, the pulsedurationintervals b'eingrnu'ch less than the pulse-(separation 'i ter'valsanw the polarity of each pulse causing said transistor]toeonducvaiid 6 store carriersduririg tlre ulse-duration;meander-storage characteristic'ofsaid transistor being suChThtitthehtbrd carriers flow out therefrom to maintain it conductive during a moch -longer portion of each subsequent p'ulse separation interval; and a load circuit coupled to said out- "10 l- -put -'-el'ectrodes =-and=-including a =eircuitresonant at said dine-scanning frequency andresponsive to said stored carriers-{for developing a sine wave- -having said frequ'ency.

-5. An a1ternating-signalegenerator comprising: a junction "transistor having er'ni'tter baseinput electrodes 'and collectoremitter output electrodes and"- eirhibitinga carrier-s'torage charaeteristic at a; predetermined frequency;

energizing means coupled to said: output electrodes gmeans coupled tosaid'input electrodes-for renderingsaid transis- '20 tor normally noncon'duc'tive; further means coupled to 's'aid-in'put electrodes for-applying theretot'periodic pulses havin'g said predetermined frequency, the pulse duration intervals being mueh less than the separation intervals mnd the polafiity-pfeach pulse causing" said transistor to "conduct and st'o're carriers during the pulse duration; the carrier storage 'eharacteristic otsaid transistor being s'u'ch that the stored carriers flow out therefrom to maintain it conductive" during a 'rnuehf longer portion of each '-'subsequent pulse separation interval; and 'aload circuit 'coupled to' said -output electro'deseand including a cireuit sonant at 'fiequney' reIated tWsaid predetermined frequencyan responsive'td said stored carriers for developing an alternating si'gnal havingsaid-resonant frequency. V

fln alternziting-si'gnali generator I comprising: 21' junction Fpo'wer transistor *having emitter-base input 1 electrodes and colleetor-emitter 'output electrodes and exh'ibitin'g -=a-- carrier-storage characteristic at a :predeterinined frequency;"energizing-means coupled to said outpufeleetrodesyineans' coupledto said-input electrodes for rendering said transist'or normally=nonconductive; further means'-'-coupled to i said input electrodes -for uapplying thereto periodiclpulses' having said. predetermined fre- C quncy, tiiepulseduration intervals being much less than -rtiie pulse separation intervals and the. polarity of 'each i pulseveausing said transistor to conduct and storecar- --ri er s;duringthe puls etdurat ion; the carrier-storage characteristidofsaid transistor beingfsueh that the stored'jearriers flow out therefrom to maintain itconductivedun mg a-rnuch longer porti on' of each subsequent pulse sep- "ar'ationintrval' iid a load circuit-coupled to said output lectrodes and ncluding" ahi'gh Q' circuit resonantwata frequency related"to"said predetermined frequency and 'res'pon'sive to said stored carriers for developing *"with substantiaPpower gain an alternating signal having said resona'nt frequency. 7-.' Asine wave' generator for'atelevision receiver c'0m- I prising: a- PNP jurietiompo'wez transistor having emitterbase input "electrodes and collector-emitter output electrodes and exhibiting -a-carrier-storage characteristic'at line-scanning -f-requency; energizing; means coupled to said-output eleetrodes;-'-nieans coupled to said input elec- ;trodes f ory rendering said transistor normally nonconddctive; further meanscoupled to said input electrodes 5 forapplying theretoi periodic.pulses having said linescanning requ nc the pu1se"di1'ration' intervals being "much lessitiian'thejpul'se' separation intervals and'each jpul'se' ljeihg of negative polarity with relation to 'said base electrode ""to eause-- 'said transistor 'to' conduct' and store carriers n" saidbase fre'g'ion during the I pulse duration;

the carrier-storagecharacteristic being such that the "stored carriers flo'w out thefefrom -to' maintain it c 'o'nductiveduring a rnueh longer portion of each subSelquentk'pulseseparation interval;- and a load circuit coutive; further -rneans eoupled tosaid input electrodes forc pled to said output eleetrodesand including a circuit resonant at said line-scanning frequency and responsive to said stored carriers for developing with substantial power gain a sine wave having said resonant frequency.

References Cited in the file of this patent UNITED STATES PATENTS 2,760,109 Schade Aug. 21, 1956 2,777,057 Pankove Jan. 8, 1957 2,787,717 Kasmir Apr. 2, 1957 10 Keller July 30, 1957 Mathis et a1. Jan. 14, 1958 Mofenson Apr. 8, 1958 Gitandhi Dec. 2, 1958 Wunderman Dec. 30, 1958 Turnage Feb. 17, 1959 Moore et al Mar. 10, 1959 Goodrich Sept. 22, 1959 Wolfendale Nov. 10, 1959 

